Regulation of sulphur and particulate emissions for diesel engines has increased in recent times. Diesel fuel which can be high in sulphur. When these diesels are nominally loaded, running at low rpms, the diesel engines emit more particulate and greater amounts of sulphur than when they are fully loaded.
When in an idle mode and low load conditions, the engine emits black carbon soot, unburned diesel fuel and causes emission of particulate oily soot. Some land-based diesel powered generator sets use a diesel particulate filter or “DPF” to reduce sulphur and particulate emissions. However, government regulations have forced all land-based users of diesel powered machines to use low sulphur diesel fuel. However, some land-based diesel generator sets continue to use high sulfur fuel.
Halyard of Dorset, U.K. makes a marine diesel particulate filter or “DPF” filter which is either turned ON manually or turned OFF manually or is subject to a programmable ON-OFF timer (a timer completely independent of engine-generator set performance).
HUG Engineering AG of Elsau, Switzerland has a marine diesel DPF system with an integral filter. The HUG system has a number of DPF filters in parallel, each with an integral heater. Diesel exhaust gas is fed into one of the parallel DPF filters (via a distribution manifold between the DPF system and the engine exhaust), but the heater is OFF in that DPF filter currently in use. A different DPF filter is selected when the filter currently in use becomes “dirty” or clogged with diesel particulate. When the in-use DPF filter is dirty-clogged, the HUG system changes the exhaust gas pathway to another in-parallel DPF filter. The heater in the dirty or clogged DPF filter is then turned ON to facilitate regeneration. The HUG system activates the filter heater for the dirty DPF filter when (a) the filter is not in use and (b) another DPF filter, in parallel to the dirty DPF filter, is currently in use. Therefore, the HUG system is an off-line regeneration system.
Existing art includes the use of porous ceramic filters which may or may not be coated with a catalyst to promote localized combustion of unburned fuel fractions and carbonaceous soot. Depending on the presence and content of a catalyst, the temperature at which combustion (known as regeneration) occurs may not be reached due to low exhaust temperature related to low engine loading.
The rate and amount of electrical power consumption on large yachts and other motor vessels is highly variable. This characteristic, in addition to the widely used practice of using the load on an electrical generator prime mover as a source of power to drive other electrical driven items, often leads to extended operation of the generator at low loads. Low loading of diesel powered generators creates a condition whereby poor combustion produces very high levels of exhaust emissions and the potential for accelerated wear of cylinder liners and piston rings.
The combination of those factors contributes to excessive soot production and the release of unburned hydrocarbons into the atmosphere. The practical impact of exhaust soot on the environment is increasingly leading to restrictions being imposed on land-based systems.
Some exhaust system manufacturers have developed and are installing diesel particulate filters (DPF) in the exhaust path to capture and oxidize soot and unburned hydrocarbons prior to their release into the atmosphere. However, because of low load operational conditions, the operational envelope of the diesel generator set leads to underloading and hence to the problems described above, DPF systems often fail due to lack of regeneration and dogging. DPFs remove soot by trapping particles within a ceramic or metallic matrix that is coated with a catalyst to promote combustion of soot particles and unburned hydrocarbons.
In order to begin combustion in a process known as regeneration the exhaust gases must reach a certain temperature. A diesel engine used to power a generator will normally produce exhaust gases at sufficient temperature when loaded at or near rated power output and the DPF will function effectively. However, when generator loads are insufficient to produce exhaust flow at the required temperature, regeneration may not occur and filter clogging will quickly result. Filter clogging results in excessive exhaust back pressure, reduced output power, and may lead to serious engine damage.
DPF filters made by DCL International Inc. of Concord, Ontario, Canada regenerate and burn the diesel particulate for low sulphur diesel fuel, namely, fuel with less than 15 ppm sulphur. The DCL filters are not useful for marine diesel fuel which typically has 1500 ppm sulphur.
Diesel Fuel Specifications: From www-DieselNet.com, the diesel fuel regulations for the U.S. specify that No. 1-D S5000 has up to 5000 ppm sulphur, the same as No. 2-D S5000. While some diesel engines use No. 2 distillate, D2069 covered four kinds of marine distillate fuels: DMX, DMA, DMB, and DMC and residual fuels (see also ISO marine fuel specifications). DMB (marine diesel oil, MDO) is allowed to have traces of residual fuel, which can be high in sulfur. This contamination with residual fuel usually occurs in the distribution process, when using the same supply means (e.g., pipelines, supply vessels) that are used for residual fuel. DMB is typically used for Category 2 (5-30 liters per cylinder) and Category 3 (greater than 30 liters per cylinder) engines. DMC is a grade that may contain residual fuel, and is often a residual fuel blend. It is similar to No. 4-D, and can be used in Category 2 and Category 3 marine diesel engines.
Highway diesel fuel in 1993 was capped at 500 ppm sulphur and then in 2006 the sulphur content was reduced by regulation to 15 ppm.
Nonroad Diesel Fuel: The following sulfur requirements are applicable to Nonroad, Locomotive and Marine (NRLM) fuels, with the exception of heavy fuel oils (HFO) used in Category 2 and Category 3 marine diesel engines. 500 ppm: Sulfur limit of 500 ppm became effective in June 2007 for nonroad, locomotive and marine fuels. 15 ppm: Sulfur limit of 15 ppm (ULSD) becomes effective in June 2010 for nonroad fuel, and in June 2012 for locomotive and marine fuels. ULSD has been legislated for nonroad engines to enable advanced emission control systems for meeting the Tier 4 nonroad emission standards.
Category 3 Marine Engine Fuel: The United States and Canada applied to the IMO to establish an emission control area (ECA) along their shorelines. Once the ECA is established, it will trigger international and US EPA sulfur limits in marine fuels: International IMO limits applicable in ECAs are 1% (10,000 ppm) sulfur beginning in 2010, and 0.1% (1,000 ppm) sulfur from 2015. SOx aftertreatment, such as SOx scrubbers, are allowed in lieu of low sulfur fuel.
US EPA 2009 EPA Category 3 marine engine rule established a sulfur limit of 1,000 ppm for marine fuels produced and/or sold for use within an ECA. SOx aftertreatment can be used in lieu of low sulfur fuel. Additional flexibilities apply to vessels operated on the Great Lakes and Saint Lawrence Seaway: the low sulfur requirements can be deferred—subject to fuel availability and economic hardship provisions—and are not applicable to steamships.